1. Field of the Invention
The present invention relates generally to a reverse common channel communication device and method for a mobile communication system, and in particular, to an RSMA (Reservation Multiple Access) communication device and method.
2. Description of the Related Art
In general, CDMA (Code Division Multiple Access) mobile communication systems have evolved from a conventional mobile communication standard which primarily provided voice service, to an IMT-200 standard which provides a high-speed data service in addition to the voice service. The current IMT-2000 standard provides high-quality voice service, moving picture service, and Internet search services. In the CDMA mobile communication system, a communication link exists between a mobile station (MS) and a base station (BS) and is divided into a forward link for transmitting a signal from the base station to the mobile station and a reverse link for transmitting signal from the mobile station to the base station.
In the conventional CDMA communication system, the mobile station uses a slotted Aloha method to access the base station over the reverse link. Prior to describing this method, a description will be made regarding the term “slots” in the context of CDMA communications. When exchanging a message in a mobile communication system, the mobile stations and base stations schedule a message transmission start time as a reference time. This reference time is called a “slot”, and a state where the reference time is scheduled is called a “slotted state”. To connect a call to a base station, a mobile station randomly selects one of a plurality of access code channels for the reverse link, and continuously sends a preamble for allowing a message transmitted over the access channel at a slot to be detected by the base station, and a data message for connection.
FIG. 1 shows access messages transmitted by a plurality of mobile stations using the slotted Aloha method.
In the slotted Aloha method, there is the possibility that two different mobile stations will transmit access channel messages at the same slot and in the same access code channel. If a base station receives the two access messages at a same slot of the access channel, the base station cannot separate nor process the two access messages. Specifically, since the mobile stations use the same frequency and PN (Pseudo Noise) code, and the same long code for the access channel, the base station cannot distinguish the access channel messages when two or more mobile stations transmit access channel messages simultaneously and at the same slot. This situation is commonly called a collision between the two messages. In this method, since the access message has a very long length of about 200 mm/s, the collision may result in much loss of data.
Referring to FIG. 2A, when a mobile station connects a call to the base station or has data to transmit over a reverse common channel, the mobile station randomly selects one of a plurality of reverse access channels and transmits a message having user information and control information over the selected channel at a slot, as shown by 213. At this point, if the base station can process the message from the mobile station, the base station transmits a channel assignment message to the mobile station over a common assignment channel at a slot, as shown by 211. Upon failure to detect the message transmitted from the mobile station over the access channel, the base station cannot send channel assignment message to the mobile station. The mobile station then retransmits the message upon failure to receive any message while monitoring the common assignment channel for a predetermined time.
In addition, when a collision happens between the messages transmitted from two different mobile stations, the base station cannot process the messages. Therefore, the base station cannot send the channel assignment message. Even in this case, the mobile stations retransmit the messages upon failure to receive any message while monitoring the common assignment channel for a predetermined time. In this situation a time delay is much shorter as compared with the slotted Aloha method since the message is very short in length.
When the base station sends a channel assignment message to the mobile station, the mobile station examines whether a channel has been assigned, and sends a message to the base station over a reverse common channel, as shown by 215. The message on the reverse common channel can be transmitted together with a pilot signal and a power control signal, as shown by 215. Upon receipt of the message, the base station performs channel estimation using the received signals in order to send power control bits (PCDs) for the reverse link over a power control channel.
FIG. 2B shows an RSMA (Reservation Multiple Access) procedure in a mobile communication system using the method of FIG. 2A.
Currently, a mobile station and a base station using the RSMA method have not been proposed in the mobile communication system. In order to communicate over a reverse common channel using the RMA method, it is necessary to construct a message to use when the mobile station sends an RSMA request to the base station. In addition, it is also necessary to construct a channel assignment message for the base station in response to the RSMA request from the mobile station, and to control the power of the reverse common channel assigned by RSMA.